The invention concerns a fixation device, as well as a method for fixation of toner material.
In electrostatic printing, a latent image is generated on the surface of a cylinder (photoconductor drum) coated with an organic photoconductor material. Toner material applied by means of a development station adheres to the latent image. The developed latent image is transferred in a subsequent step to a printed stock surface guided along the photoconductor drum. Another variant transfers the developed latent image first to an intermediate support and from it to the printed stock surface. The developed latent image is imaged on the printed stock surface because of this.
The developed image is fixed by means of a fixation process on the stock surface. Ordinary methods are then heating roll fixation, flash lamp fixation and fixation by means of a heat radiation source.
Various solution proposals of fixation devices for drying and fixation of the toner material on the stock surface have therefore been offered.
The European Patent Application EP 0 992 864 discloses fixation of ink on a sheet-like and/or endless support, especially toner powder on paper, in which the ink coating is heated in order to achieve melting of the toner and permanent bonding with the support. The inking is irradiated with infrared radiation, especially from an infrared lamp, at emission temperatures of 2500 K or higher, so that the ink is heated by absorption of at least part of the infrared radiation and fixed.
In unheated stock, the problem of blister formation in the toner material exists. Previous devices were therefore offered that preheat the stock, i.e., the stock is heated before the fixation process.
A shortcoming here, in the first place, is the space requirement for the additional preheater and, on the other hand, the energy loss by transport from the additional preheater to the radiation unit. Another shortcoming is that fixation of the toner material does not occur completely and sufficiently with inadequate toner material viscosity as a function of temperature.
One task of the invention is therefore to provide a fixation device and method for efficient fixation of toner material, regardless of the toner density of the toner material.
The invention discloses a fixation device for fixation of toner material for an electrophotographic printer, in which the toner material can be exposed and fixed by a radiation unit and the stock, as support of the toner material, can be heated essentially during exposure by the device. Since the stock, during exposure by the device, is heatable, the fixation device is compact, energy- and space-saving. The full range of toner densities from 10% to 400% can be appropriately fixed with the fixation device according to the invention. The toner density here refers to the density of the toner layer on the stock, which, in one-color printers, ordinarily ranges from 10% to 100%. A toner density of 400% is obtained, for example, from four toner layers of different color, each with 100% density of each toner layer.
The device can contain at least one microwave applicator for microwave radiation, through whose use the fixation process is more rapid and more economical as part of the printing process and, as a result, the entire printing process is accelerated. The energy utilization during application of a microwave applicator is high, since the microwave energy is almost exclusively used to heat the stock. This phenomenon also causes good controllability of the heating process. The radiation unit is separated from the microwave field produced by the microwave applicator by a screen.
It has turned out to be particularly advantageous for the fixation process if the toner material, during the exposure process, lies above the glass transition point of the toner material and the stock below this glass transition point, for example, with a temperature of the toner material of 110xc2x0 C. and a temperature of the stock of 60xc2x0 C. When this condition exists, the temperature gradient is low and the toner material maintains the temperature above the glass transition point over a relatively long period, so that advantageous effects are achieved in printing quality.
Exposure can occur with high energy density with radiation essentially in the ultraviolet spectral region. Ultraviolet radiation has the particular advantage here that it is absorbed in very high percentage by the toner material being exposed, independently of the color of the toner material, but, on the other hand, is almost completely reflected by the stock. The energy utilization is therefore higher in comparison with radiation of other frequencies.
The device can also contain a cooling device, with which a specific temperature gradient can be produced between the stock and the toner material for rapid and reliable fixation. The cooling device, during double-sided printing, further serves to avoid melting of the toner material from one side of the stock, while the toner on the other side is exposed and the stock, as described, is heated. Moreover, the stackability of the stock is guaranteed by the cooling device, since, during stacking of heated stock after application of the toner material, the printing stock sheets can stick to each other and wipe off the toner material.
For further energy utilization, the radiation unit is allocated at least one resonator, in which a microwave field is formed, and which leads to uniform homogenous distribution of the microwaves within the resonator housing and thus to uniform homogenous heating of the stock.
In a modification, additional resonators can be arranged along a transport path of the stock, so that the wave maxima of the electromagnetic fields of the individual resonators are situated essentially along the stock. The wave maxima are shifted here relative to each other by the spacing xcex/(2*number of resonators). The stock is heated in the individual resonators in regions so that, after the stock has passed through all the resonators, a roughly uniform heating of the stock is performed. Thus, the stock is roughly half heated during passage through the first resonator periodically according to the wave maxima, when two resonators are used for the fixation device, and, on passage through the second resonator, the other half of the stock is heated.